Because of the developments in optical amplifiers and signal modulation/demodulation schemes, optical communication transmission capacity has increased rapidly. Also, because of the growth in Fiber-To-The-Home (FTTH), data demand has also increased, and it is necessary to increase transmission capacity even further in the future. One of the effective ways to increase transmission capacity is to increase the bandwidth of the used transmission wavelength band. And, as a way to increase the bandwidth, a holey fiber, which is a new type of an optical fiber having hole structure, can be used for broadband optical transmission. For example, K. Jeda, K. Kurokawa, K. Tajima and K. Nakajima, “Visible to infrared high-speed WDM transmission over PCF,” IEICE Electron Express, vol. 4, no. 12, pp. 375-379 (2007), discloses a technique to perform 658˜1556 nm broadband optical transmission over a transmission length of 1 km using a Photonic Crystal Fiber (PCF), which is one kind of the holey fiber. Also, with respect to the holey fiber, transmission loss and fiber length are also improved (see, for example, K. Kurokawa, K. Tajima, K. Tsujikawa, K. Nakajima, T. Matsui, I. Sankawa and T. Haibara, “Penalty-Free Dispersion-Managed Soliton Transmission Over a 100-km Low-Loss PCF,” J. Lightwave Technol., vol. 24, no. 1, pp. 32-37 (2006), and K. Tajima, “Low loss PCF by reduction of holes surface imperfection,” ECOC 2007, PDS2.1 (2007)). For example, Tajima et al. discloses a holey fiber with a relatively low transmission loss of 0.18 dB/km at a wavelength of 1550 nm. As stated above, broadband optical transmission using holey fiber has a large potential for commercialization in the near future.
Characteristics of the holey fiber are primarily based on the hole diameter (d), the distance between the closest holes (Λ), and the ratio of the two (d/Λ). According to M. Koshiba and K. Saitoh, “Applicability of classical optical fiber theories to holey fibers,” Opt. Lett., vol. 29, no. 15, pp. 1739-1741 (2004), if holes in a holey fiber are arranged in a triangular lattice shape, theoretically all wavelengths are transmitted in single-mode when d/Λ is less than 0.43. The characteristic that enable single-mode transmission at all wavelengths is called Endlessly Single Modes (ESM). If single-mode transmission is realized in this manner, then much faster optical transmission is possible. At the same time, a coupling of a light with a higher-order mode of the holey fiber can be prevented when the light is inputted into the holey fiber through another optical fiber and alike, which are connected to the holey fiber, thus preventing an increase of a connection loss. Even if d/Λ is approximately 0.5 as shown in Ieda et al., if the length of the optical fiber is, for example, longer than 1 km, then higher-order-modes are attenuated during transmission, and therefore the optical fiber effectively achieves ESM characteristics.
As a type of the holey fiber, a multi-core holey fiber having a plurality of cores arranged separately from each other is disclosed (see PCT WO 2006/100488). Because this multi-core holey fiber can transmit a different optical signal through each of the cores, for example, it is considered to enable an ultra-high capacity transmission by way of a space division multiplexing (SDM) transmission.
However, multi-core holey fibers experience crosstalk deterioration between optical signals because interference among optical signals in a plurality of cores causes some portion of the transmitted optical signal in one core to leak into another core(s).